The present invention relates to fiber optic sensors. In particular, it relates to an interferometric sensor that is capable of making simultaneous measurements for a variety of environmental conditions.
The simultaneous measurement of various environmental conditions such as temperature and pressure is critical for numerous aerospace, medical, industrial, and automotive applications. In many instances, a temperature measurement is needed to compensate for the thermal sensitivity of a pressure sensor. This is especially true for optical fiber pressure sensors where high operating temperatures and high sensitivity measurement is needed.
Several approaches have been used to thermally compensate optical fiber pressure sensors. One approach is to co-locate separate pressure and temperature sensors in the same environment. The reading from the temperature sensor can then be used to correct for temperature induced pressure measurement error which has been previously characterized. However, this approach has certain limitations in that it necessitates multiple fibers and/or leads, complex signal processing, and is an inexact measurement of the pressure sensor""s temperature.
Herman et al. (U.S. Pat. No. 5,414,507) describe a mechanical design to compensate for temperature. Their fiber optic pressure sensor is a transducer that utilizes fiber optics in an interferometric sensing element. In this arrangement, a temperature compensator is attached to the sensing or reference optical fiber of the interferometer. The temperature compensator causes corrective changes in the length of the optical fiber to which it is attached, thereby compensating for opposing temperature effects on other parts of the interferometer. A bellows-type pressure-to-movement converter translates sensed pressure into linear displacement. That displacement is mechanically coupled to, and thereby varies the length of, one leg of a fiber optic interferometer as a function of pressure. The disadvantage of this type of sensor arrangement is that it is limited to applications where larger, less accurate measurement is acceptable.
Maron (U.S. Pat. No. 5,844,667) discloses a temperature compensated intrinsic optical fiber pressure sensing device. The intrinsic fiber optic sensor is formed in the core of an optical fiber. A diaphragm that is responsive to pressure in an environment applies a longitudinal strain in the optical fiber, thereby inducing a pressure stress in the intrinsic fiber optic sensor. The intrinsic fiber optic sensor responds to a pressure stress that provides a sensing light signal indicative of the pressure. Temperature compensation members respond to temperature through an applied longitudinal strain in the optical fiber, thereby inducing a temperature compensation stress in the intrinsic fiber optic sensor indicative of the temperature. Changes in the sensing light signal that are attributable to changes in the temperature compensation strain substantially compensate for changes in the sensing light signal attributable to changes in the temperature of the intrinsic fiber optic sensor. This type of sensor has its limitations in that the measurement obtained is limited to applications where larger, less accurate measurement is acceptable.
Maron (U.S. Pat. No. 6,016,702) describes a pressure sensor that includes at least one intrinsic fiber optic sensor element formed within a core of an optical fiber. A temperature compensation sensor is also formed in the fiber near the location of the pressure sensor or alternatively, temperature compensation is provided by an intrinsic fiber optic sensor element mounted to experience an equal but opposite strain associated with changes in the dimension of the pressure sensitive structure. In particular, the pressure sensor utilizes resonant structures or Bragg gratings that are disposed at one or more locations within the waveguiding core of an optical fiber. Bragg gratings are optical gratings written into a fiber for highly multiplexed measurement of strain and temperature. By using a complicated sensor housing design, changes in pressure induce strain in the fiber which is measured with two multiplexed Bragg gratings. Each Bragg grating sensor measures the strain and temperature seen by the optical fiber. One of the sensors is mechanically coupled to the housing which converts changes in pressure to strain on the fiber, while the other sensor is strain isolated to measure only temperature. The challenge of this design is the complicated housing required to minimize the extensive cross-sensitivity of the sensors.
These prior approaches have many limitations. Any approach that uses a mechanical means for thermally compensating pressure is limited in size, operating temperature, accuracy, commercialization, as well as it does not actually provide a temperature measurement. Approaches that use an electrical thermocouple and an optical fiber sensor suffer from the traditional limitations of electrical based gages, which is often why optical fiber gages are used in the first place. Approaches that employ multiple optical fiber sensors to measure temperature and pressure often require multiple leads and are limited in their effectiveness for thermal compensation because the sensors are not a single device. In addition, approaches that use Bragg gratings are typically limited to operating temperatures below 1000xc2x0 C.
Extrinsic Fabry-Perot interferometeric (EFPI) sensors are based on a combination of two light waves and are described in U.S. Pat. No. 5,301,001 to Murphy et al. which is incorporated by reference herein. A typical EFPI sensor consists of a single-mode input fiber and a reflector fiber aligned by a hollow core silica tube. The operation of an EFPI can be approximated as a two beam interferometer. When the laser diode light arrives at the source fiber end-face, a portion is reflected off the fiber/air interface (R1) and the remaining light propagates through an air gap (L) with a second reflection occurring at the air/fiber interface (R2). In an interferometric sense, R1 is the reference reflection and R2 is the sensing reflection. These reflective signals interfere constructively or destructively based on wavelength and the optical path length difference between the reference and sensing fibers. Small movements in the hollow core cause a change in the gap length, which changes the phase difference between the sensing and reflecting waves producing fringes.
Haritonidis et al. (U.S. Pat. No. 4,942,767) describe one form of a fiber optic sensor where a micromachined diaphragm is positioned across a gap from an end of an optical fiber. The end of the optical fiber provides a local reference plane which splits light carried through the fiber toward the diaphragm. The light is split into a transmitted part which is subsequently reflected from the diaphragm, and a locally reflected part which interferes with the subsequently diaphragm reflected part. The interference of the two reflective parts forms an interference light pattern carried back through the fiber to a light detector. The interference pattern provides an indication of diaphragm deflection as a function of applied pressure across the exposed side of the diaphragm. To detect the magnitude and direction of diaphragm deflection, a second fiber is positioned across the gap from the diaphragm. The second fiber provides an interference pattern in the same manner as the first fiber but with a phase shift. This device has several shortcomings. In particular, the diaphragm must be completely reflective in order to obtain an accurate measurement. In addition, there is no way to compensate for temperature effects on the sensor.
An object of the present invention is to provide an interferometric sensor that can be used to monitor a plurality of environmental conditions from the same location at the same time.
Another object of the present invention is to provide an interferometric sensor that employs a single optical fiber or sensor lead.
Another object of the present invention is to provide an interferometric sensor having a flexible sensor platform.
By the present invention, an interferometric sensor is provided. The sensor may be used for various medical, automotive, aerospace, and process monitoring applications. The interferometric sensor comprises an optical fiber and a plurality of sensing regions positioned in an operable relationship to the optical fiber. Each sensing region has partially reflective boundaries and is capable of producing an interferometric signal. The interferometric sensor may operate as an extrinsic, intrinsic, or combination extrinsic/intrinsic sensor depending on the desired application.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention will be obtained by means of instrumentalities in combinations particularly pointed out in the appended claims.